Timing adjustment is limited to setting point gap on these engines if the stator plate (the thing the points are mounted on) cannot be moved and the 'cam' on the cranksaft that the point's lever/pad rides on is in a fixed position.
I don't see how boring the cylinder and a piston change would change timing.. but who knows.. maybe something got tweaked during the process.
Another possibility that could explain symptoms of a pre-ignition type "ping" is a high compression ratio. This could be the result of a piston change if the new piston crown protrudes further into the combustion chamber than the old piston did.
Another possibility is compression ratio was raised by whoever did the cylinder work.. perhaps they cut something off the top of the cylinder, shortening it. Another possibility is the cylinder base gasket is missing (?) or needs to be thicker.
To test the possibility that compression ratio is too high, use a very thick head gasket or more than one head gasket. Use something near a total thickness of about one millimeter more than what is now there. This will lower compression ratio by a bunch, adding about 1.2ccs to the volume of the combustion chamber. If the pinging stops, you're on the right track..
This lower compression ratio might stop the ping.. however it could do this by just hiding and masking the source of the problem.. a misadjusted, very advanced ignition timing. In other words, it might not ping anymore but compression ratio was not the source of the ping.. excessive heat due to advanced ignition timing was.
So, in any case, check timing with a timing light or with the depth gauge method through the plug hole, or however the owner's manual instructs.